Heretofore, N(mercaptomethyl) phthalimide S-(O,O-dimethyl phosphorodithioate), see U.S. Pat. No. 2,767,194, was produced in a batch process by reaction between N-chloromethyl phthalimide, hereinafter referred to as CMP, and sodium O,O-dimethyl dithiophosphate, hereinafter referred to as dithioacid salt, as follows: ##STR1##
The CMP in the above reaction is formed by reaction between N-hydroxymethyl phthalimide, hereinafter referred to as HMP, and hydrochloric acid, as follows: ##STR2##
Reactions (1) and (2) are carried out in a batch process. Reaction (2) is carried out in a first reactor. The CMP produced is transferred to a second reactor where Reaction (1) is carried out.
The HMP is supplied to the first reactor in the form of a wet filter cake. The moisture in the filter cake reduces the strength of the aqueous HCl. The acid strength is fortified by the addition of anhydrous HCl, bubbled through the reaction mixture. The reaction occurs in the presence of an organic solvent such as benzene in an amount sufficient to dissolve the CMP produced in the reactor. The CMP-containing organic phase is then separated from the aqueous phase and transferred to the second reactor where it reacts with the dithioacid salt to form the phosphorodithioate ester. The product of the reaction which occurs in the second reactor is a two-phase mixture containing the phosphorodithioate ester in the lighter organic phase and sodium chloride and other aqueous wastes in the heavier aqueous phase. The phosphorodithioate ester is obtained from this mixture by phase separation followed by caustic and water washes and solvent stripping.
The procedure for the above-described process is as follows:
1. The first reactor is charged with wet HMP cake. PA1 2. 35% aqueous HCl is added at a quantity of about 50 gallons per lb - mole of HMP. PA1 3. benzene is added at a quantity of about 130 gallons per lb - mole of HMP. PA1 4. anhydrous HCl is bubbled through the reaction mixture at a rate of from about 15 to about 25 lb per hour per lb - mole HMP charged in Step (1). As the HCl is bubbled through the mixture, the temperature is maintained at 45.degree. C by the application of cooling water. The reaction is monitored by checking the acid strength which rises at the completion of the reaction when HMP is no longer present to consume the HCl. PA1 5. At the completion of the reaction, the aqueous HCl phase is removed. PA1 6. The benzene solution of CMP is pumped together with the dithioacid salt to the second reactor where the reaction to form the phosphorodithioate ester occurs. The reaction is conducted at about 70.degree. C and about atmospheric pressure. PA1 7. The contents of the second reactor are phase separated, the product phase is washed with caustic and water, and the solvent is stripped from the product. PA1 1. The production capacities of both reactors can be increased with no change in the size of either vessel and with no increase in the overall batch cycle time, and PA1 2. The solvent stripping load and loss per unit of product in the purification section can be reduced, conserving heat energy.